29 Jan 2021

How to take an accurate wear down measurement

During a survey, it is required by class to take a wear down measurement at the aft stern tube seal. This will normally be carried out during dry-docking. However, if the fitted stern seal is adapted to allow this, it is also possible to take measurements when the ship is still in the water. This makes it possible to execute an in-water survey for class.

There are different reasons for taking a wear down measurement. The most important one is to get an indication of the wear and tear of the stern tube bearing. The measurement taken will be compared with the previous reading. Previous readings are normally registered in a service report as well as on a form in the box containing the poker gauge.

As class requires a reference reading, it is important that every reading is taken under the same conditions. This means always using the same tool, always having the shaft in the same position and of course always measuring from the same location within the seal. The plug hole that must be used for the wear down measurement is normally indicated on the drawing in the manual. Sometimes, the subject plug is also marked.

The position of the shaft is often marked on the propeller. However, some may argue that it is more reliable to use a fixed position of the engine as a reference position, e.g. piston 1 in top. Whatever the case, the reference position should be stated in the poker gauge toolbox.

Measurements are taken both at the top and at the bottom of the seal. As the total differences are fixed, a change in reading of, for example, 0.1 mm at the top, should reduce the reading at the bottom by the same amount. If not, check the plug holes again; the copper sealing ring is probably still inside the plug hole, making the reading useless.

Additional attention must be paid before and after the complete removal of the aft stern tube seal. As the seal is fitted with a spigot into the stern tube flange, readings can differ before and after the repair. This because the spigot has a certain clearance with the flange. The seal housing needs to be fitted and checked with a feeler gauge for equal clearance to the shaft liner. New readings need to be recorded and will serve as the reference point for the next wear down measurement.

If the new reading indicates a considerable difference compared to the previous reading, this indicates stern tube bearing wear. The limits of this wear depend on the type of bearing and the limits set by the manufacturer. In case of suspected excessive wear, further investigation by a specialised engineer is required.

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